The phrase "fundamental constituents" refers to the basic building blocks or essential elements of something. It represents the most basic parts or components that make up an object or system. Full definition
That was the last time particles called quarks and gluons — the most fundamental constituents of matter — roamed free in the cosmos, and it was a brief run. (discovermagazine.com)
According to Whitehead something analogous to what we experience as mentality, something like «feeling» or «perception,» is present throughout the natural world, not just in man, animals and plants, but also in the most fundamental constituents of the physical world. (religion-online.org)
TECHNICOLOR Perhaps the Higgs is not a truly elementary particle but a bundle of more fundamental constituents, much as the proton is a mini galaxy of quarks and gluons. (scientificamerican.com)